A polyamide resin represented by nylon 6 and nylon 66 is excellent in toughness, chemical resistance, electric properties, etc. and widely used as the molding materials for automotive parts, machine parts, electrical or electronic equipment parts, etc. Of the polyamide resin, a polyamide produced from m-xylylenediamine and adipic acid (may be referred to as “nylon MXD6”) is characterized by its high strength, high elasticity, and low water absorption, as compared with the conventional polyamide resin, and comes to be used as a substitute for a metallic material in the production of electronic equipment parts and automotive parts which require light weight and small size. Therefore, the demand for nylon MXD6 is escalating.
Since the crystallization speed of nylon MXD6 is low as compared with those of nylon 6 and nylon 66, nylon MXD6, when used singly, is difficult to crystallize in an injection mold, making the thin-wall molding difficult and causing problems of deformation of molded article and lowering of mechanical strength. To use nylon MXD6 as a molding material while avoiding such disadvantage, Patent Document 1 proposes to increase the crystallization speed of nylon MXD6 by blending nylon 66 or talc powder each having a high crystallization speed or proposes to improve the moldability of nylon MXD6 by increasing the mold temperature. Since the water absorption of a nylon MXD6 composition blended with nylon 66 is higher than that of nylon MXD6 alone, the mechanical strength is reduced by water absorption.
To solve the above problem, Patent Documents 2 and 3 propose to blend inorganic filler with a copolyamide having a high crystallization speed which is produced by using, as the main raw material, a mixture of m-xylylenediamine and p-xylylenediamine as the diamine component and adipic acid as the dicarboxylic acid component. The thin-wall molding, which has been difficult by the use of a conventional molding material containing nylon MXD6, has been made easy by the proposed method. The proposed method makes improvement also in shortening the molding cycle, lowering the mold temperature, and preventing the mechanical properties from being lowered due to water absorption. However, a copolyamide with a uniform high quality has been difficult to obtain by a known method of producing a copolyamide using p-xylylenediamine, because a high melting point copolyamide with a high content of p-xylylenediamine is formed locally, although its amount is very small. The copolyamide with a high p-xylylenediamine content locally formed does not dissolve in the system at the production temperature and granularly disperses throughout the copolyamide being produced without increasing its polymerization degree. A molded article made of the resultant heterogeneous polyamide has uneven mechanical properties and heat resistance due to the dispersed high melting point polyamide.
In a common batchwise production method of polyamide using a nylon salt or its aqueous solution, the aqueous solution of nylon salt is heated under pressure in a single reaction vessel to allow the polymerization to proceed in a homogeneous phase while preventing the diamine component from escaping by evaporation. After fixing the diamine component, the water vapor in the system is gradually released to reduce the pressure finally to atmospheric pressure or reduced pressure, thereby completing the polymerization. In this method, since an aqueous solution of nylon salt with a concentration of about 50 wt % is generally used, a large amount of solvent water and the condensation water eliminated by condensation must be removed. Therefore, a countermeasure should be taken against various disadvantages, such as foaming, solidification of polymer due to the evaporation latent heat of water, heat degradation of polyamide which is adhered to the inner wall of reaction vessel due to a large change of liquid level during the reaction. In addition, this method requires a large amount of heat energy for removing a large amount of water and the single batch yield of polyamide is small. Therefore, this method involves many technical and economical problems. A uniform copolyamide including p-xylylenediamine is also difficult to produce by this method, because the polyamide being produced adheres to the inner wall due to a large change of liquid level and the high melting point polyamide copolymerized with p-xylylenediamine is easily deposited on a stirring shaft having a low heat transfer efficiency during the removal of a large amount of water due to the water returned to the reaction apparatus and its latent heat of evaporation.
As a polymerization method without using a nylon salt or its aqueous solution, Patent Document 4 discloses a method of allowing the reaction to proceed by adding a diamine component containing a small amount of water dropwise to a dicarboxylic acid component at 220° C. under atmospheric pressure. In the method disclosed in Patent Document 5, a mixture of a dicarboxylic acid and a diamine is subjected to polycondensation reaction under atmospheric pressure at a reaction initiation temperature equal to or higher than the melting point of the dicarboxylic acid. The reaction is allowed to proceed by elevating the temperature so as to keep the reaction system containing the raw mixture in substantially uniform molten state. Before the reaction rate reaches 95%, the temperature of the reaction system is raised to a temperature equal to or higher than MP −30° C. wherein MP is the melting point of the copolyamide being formed, thereby controlling the reaction temperature so as to allow the reaction to proceed in homogeneous system without losing its fluidity. These methods are characterized in that the diamine is continuously added to the reaction system while maintaining the reaction system which contains oligoamide and polyamide being formed in uniform molten state. Since the reaction is carried out under atmospheric pressure, the apparatus for the polycondensation reaction is not needed to be pressure-resistant.
As compared with the known production method of polyamide, this method extremely cuts down the time required for the polycondensation reaction, because the operation for increasing or decreasing the pressure of system and the removal of water used as the solvent, which is inevitable in the method using an aqueous solution, are not needed. In addition, this method is very advantageous for the production of copolyamide, because the method requires no heat for concentrating the aqueous solution, and the amount of charge for a single reaction can be increased, to enhance the productivity. However, if the production of a copolyamide including p-xylylenediamine is carried out by this method, the problem of local formation or accumulation of a copolyamide with a high content of p-xylylenediamine becomes more considerable.
In the above production method, the dicarboxylic acid evaporated from the molten dicarboxylic acid deposits on the inner wall of the polymerization apparatus and reacts with the diamine evaporated in the same manner, to form a nylon salt or oligomer. As compared with a salt between m-xylylenediamine and a dicarboxylic acid, since a salt between p-xylylenediamine and a dicarboxylic acid is less soluble in water and also hardly dissolves in the condensation water eliminated by the reaction and returning to the reaction system, the nylon salt including p-xylylenediamine is likely to selectively accumulate. The accumulated nylon salt undergoes the amidation in a solid state and converts into a water-insoluble, high melting point oligomer. If the accumulated oligomer falls away from the wall and enters into the reaction liquid at earlier stage of the production, the oligomer is depolymerized with the reaction liquid before the completion of reaction, to give a uniform product. If enters into the reaction liquid at later stage of the production, since the oligomer is not mixed well with a highly viscous reaction liquid and is not depolymerized completely, the oligomer remains in the polyamide as a high melting point foreign matter. The accumulated product exposed to heat for a long time after repeating the batchwise production becomes a gel material which is insoluble in polyamide, likely to cause uneven properties and deteriorated properties.
Of the parts of a polymerization apparatus, the nylon salt or oligomer mainly accumulates in a pipe for introducing the vapor mainly comprising the condensation water eliminated by the polymerization from a reaction tank to a partial condenser and accumulates in the partial condenser. If continuously accumulated, the pipe and partial condenser is clogged and the batchwise production cannot be continuously repeated. In the production of a polyamide from a diamine and a dicarboxylic acid, it is very important to control the mole balance to achieve a desired degree of polymerization. Since the amount of accumulation in the reaction tank varies batch to batch, it is difficult to control the mole balance precisely. Thus, the production method in which a diamine component is added to a dicarboxylic acid component under atmospheric pressure involves many disadvantages for the production of uniform and satisfactory products.
Patent Document 6 discloses to add a whole amount of diamine to a dicarboxylic acid in an extremely short time and allow the reaction to proceed under pressure. The proposed method involves various disadvantages, which are attributable to the addition of a whole amount of diamine in an extremely short time. Since a large amount of condensation water is eliminated in a short time, a countermeasure should be taken against foaming, change of liquid level, solidification of polymer due to the evaporation latent heat of water, and escape of monomers by evaporation. Particularly, a high pressure is needed. Therefore, it takes a long time to reduce the pressure for allowing the reaction to proceed, because the pressure should be reduced while preventing foaming. During this operation, the polyamide is exposed to high temperature for a long time, this promoting the oxidative degradation of polyamide molecules to cause the yellowing. In addition, a large amount of heat energy is required in a short time for removing a large amount of the condensation water which is eliminated in a short time and maintaining the reaction system at a temperature enough to keep the whole reaction system in fluid condition, thereby requiring a heating means excessively large in view of the amount of the produced polyamide. Thus, the proposed method involves many technical and economical problems.
Patent Document 7 discloses a method in which a diamine component comprising m-xylylenediamine and p-xylylenediamine is added to adipic acid, while decreasing the concentration of p-xylylenediamine in the diamine component in a later stage of the reaction. The proposed method uses different diamine components containing the ingredients in different proportions, thereby increasing the number of equipments to be used. In addition, the diamine component being adding is changed to another during the reaction, this complicating the operation. Thus, the proposed method cannot be said as an efficient method. Since the concentration of p-xylylenediamine in the diamine component used in an earlier stage of the addition is high, the nylon salt and oligomer mentioned above are likely to accumulate in the apparatus. Therefore, the uneven properties due to the inclusion of a high melting point copolyamide become more remarkable. As described above, it has been demanded to provide a copolyamide which is produced from a diamine component including p-xylylenediamine and a dicarboxylic acid component and has excellent properties and stable quality.